Gas turbine components that are exposed to hot combustion gases are often coated with metallic and/or ceramic coatings to improve their heat resistance and thereby their lifetime and their operational behaviour. Examples of such coatings are MCrAlX-coatings where M stands for a metal, in particular for cobalt, nickel or iron, X stands for an active element which can, for example, be yttrium (Y) and/or silicon (Si), scandium (Sc) and/or at least one rare earth element or hafnium (Hf). Such alloys are, for example, known from EP 0 486 489 B 1, EP 0 786 0 17 B1, EP 0 412 397 B1 or EP 1 306 454 A1. A thermal barrier coating (TBC) may be applied on the MCrAlY-coating. Such thermal barrier coatings are typically made from zirconium oxide (ZrO2) the crystal structure of which may be partially or completely stabilised by adding yttrium oxide (Y2O3).
Furthermore, turbine components which are exposed to hot combustion gases often comprise at least one internal cavity and cooling holes extending from the internal cavity to the outside of the component. In operation of such a component a cooling fluid, e.g. cooling air, is blown out of the cooling holes to form a cooling fluid film over the component's surface to protect it from hot combustion gas.
Metallic alloy coatings or ceramic coatings are usually applied onto the surface of the component by thermal spraying methods like, e.g. atmospheric plasma spraying (APS) or high velocity oxy-fuel spraying (HVOF). In any case, during the application of the coatings the cooling holes can become reduced in their cross section area or even completely clogged. As a consequence, the cooling fluid flow efficiency through the cooling holes is reduced. Partially or fully clogged cooling holes therefore need to be reopened after the coating is applied. However, reopening of the cooling holes is a time consuming and elaborate and therefore an expensive process. In addition, cooling holes are often formed by electrical discharge machining (EDM) which cannot be used after coating the component with an electrically isolating ceramic coating.
Due to the mentioned difficulties it has been proposed to mask the cooling holes during the coating process. EP 1 365 039 A1 describes a process of masking cooling holes of a gas turbine component in which the cooling holes are filled with a polymer as masking material which can be thickened by using ultraviolet light. After thickening the masking material the coating is applied. After applying the coating remaining masking material in the cooling holes may be removed by burning it off. DE 10 2006 029 070 B3 describes a method for coating a component with cooling holes in which the cooling holes are masked during the coating process. A particle beam is used for coating the component. The masking material is chosen such that it is carried off by the particle beam.